1. Field of the Invention
The invention concerns a seal plate in the internal air system of a gas turbine engine.
2. Description of Related Art
A gas turbine engine internal air system does not contribute directly to engine thrust but has several important functions to perform for safe and efficient operation of the engine. Chief among these functions is cooling of static and rotary stages including vanes, blades, discs etc, control of turbine tip clearances and prevention of hot gas ingestion into, for example, turbine disc cavities. Up to about one fifth of total engine core mass flow may be diverted into this internal air system through bleed outlet at one or more locations in the compressor system. Consequently work has already been done on air consumed by the internal air system in compressing it. Leakage losses are therefore a total loss to the engine and have a negative effect on thrust and engine efficiency.
Seals between relatively static and rotating engine stages represent escape paths for the system air and ingenuity and effort is directed at reducing such losses in order to mininise the drain of compressed air and as one way of raising engine efficiency. In an internally cooled turbine stage, it is found desirable to have a low-leakage air seal at a high radius, essentially just radially inboard of the turbine disc rim. The seal helps define a plenum chamber bounded on one side by a face of the turbine disc itself from which turbine blade internal cooling air is drawn. In passing through the plenum the air also passes over the disc face and helps cool it.
It has been found advantageous in these circumstances to use an air riding seal or face seal of the kind in which a relatively stationary ring or collar is maintained in close proximity to a relatively rotating face plate. In effect the ring rides on a cushion of air without coming into rubbing contact with the plate maintained by a balance of axially directed forces. In such an arrangement it is necessary to maintain an accurate alignment between the confronting faces of the relatively rotating ring and plate.